The present invention involves porous metal-containing materials, a method for making porous metal-containing materials, and products and uses involving the porous metal-containing materials.
Porous metal materials present a broad spectrum of possibilities for advanced applications. In the case of metallic foams, applications include filters, battery and fuel cell electrodes, heat exchangers, catalysts and light-weight structural materials. The advantages of porous metal materials generally relate to high porosity, high surface area and light weight. For many applications, it is desirable that the porosity be as high as possible. Another desirable feature, for many applications, is that the pore volume within the porous metal be highly interconnected, or open. This is particularly true for metallic filters, electrode materials, heat exchangers and catalysts.
Metallic foams have been produced by liquid and solid state processing methods. A foaming gas can be used to create the pore volume in metallic foam materials, but the major limitation of that method is that a predominantly closed pore structure often results. Metallic foams can also be produced through the addition of a surrogate pore-former, typically an organic material, that is removed, either thermally or chemically, after fabrication. Also, metallic foams have been fabricated by deposition or infiltration of a metal into open-cell polymer foam substrates. Loose sintering of fine metallic powders has been used to produce structures with densities in the range of 30 to 50%, but control of the resulting microstructure can be very difficult. Porous metals used as filters have been made by partial sintering of compacted metal powders. The porous metals made by partial sintering, however, typically have densities greater than 50% and poor permeability characteristics.
Takahar and Fukuura, in U.S. Pat. No. 5,417,917, describe a method for partially sintering certain particulate metal oxides in air, followed by reduction to a porous metallic form. They report that porosities of over 60% have been obtained with nickel. There are, however, problems with the method described by Takahar and Fukuura.
A significant problem is that the method is reported to involve a high level of shrinkage during processing. Therefore, to obtain relatively high porosities in the final porous metal structure, significant amounts of organic material are mixed with the starting metal oxide powders and the powders are only lightly compacted prior to sintering. Under these conditions, it is doubtful that extensive joining of grains will develop during the sintering step, with a result being that the porous metal structure following reduction may not have a high structural strength, severely limiting the potential applications for the material. Moreover, a low structural strength could render the material susceptible to particle shedding, which would be particularly undesirable for filter applications.
Furthermore, the high level of shrinkage reported by Takahar and Fukuura significantly limits the ultimate porosity that may be achieved in the final product, even with the addition of organic material and with only light compaction of starting powders. Furthermore, addition of organic material can result in residual organics remaining in the final product, which could contribute to detrimental outgassing if the material were used in high performance air filter applications, such as filtering air for xe2x80x9cclean roomsxe2x80x9d required for certain semiconductor fabrication and pharmaceutical manufacture operations. Moreover, a high level of shrinkage may actually result in the closing of some of the pore space during processing, reducing the permeability of the material.
There is a need for improved porous metallic materials and methods for manufacturing such materials.
With the present invention, it has been found that porous metallic materials can be made by sintering a green form metal oxide followed by chemical reduction to a metallic form and with only a low, or negligible, level of shrinkage during processing, provided that the sintering step is conducted under conditions to promote vapor phase sintering. It has also been found that such porous metallic materials may thus be made to include very high porosities and a desirable open pore structure with a small mean pore size.
In one aspect, the present invention provides a porous metal-containing material having highly interconnected, open pore volume. Depending upon the specific material and the characteristics of the final porous metallic product, especially the pore size, the porous metallic material may have a porosity of 40 volume percent or larger. The porosity for most applications, however, will typically be larger than about 60 volume percent and more typically larger than about 65 volume percent. In a preferred embodiment, the porosity is larger than about 70 volume percent.
Nonlimiting examples of some metals that may be included in the porous metallic materials include iron (Fe), nickel (Ni), copper (Cu), vanadium (V), cobalt (Co), zinc (Zn), cadmium (Cd), tin (Sn), tungsten (W), chromium (Cr), niobium (Nb) and molybdenum (Mo). The porous metal-containing material may include substantially only a single metal, or may include more than one metal.
The porous metal-containing material may be made with any convenient pore size, which is typically smaller than about 100 microns. For many applications, however, the pore size will be smaller than about 10 microns, smaller than about 5 microns, smaller than about 2 microns, or even smaller than about 1 micron for some applications.
The pore volume of the porous metal-containing material is highly interconnected, so that most, and preferably substantially all, of the pore volume is open. There is very little, and preferably substantially no, closed pore volume. Because of the highly interconnected nature of the pore volume in the porous metal-containing materials, the materials typically exhibit a very high permeability to fluids. The high permeability of the porous metal-containing materials is particularly desirable for applications involving fluid flow, such as filtration and heat exchange.
In a major aspect, the present invention provides a method for manufacturing porous metal-containing materials. The manufacture method is particularly advantageous for making extremely high quality metal-containing materials with a high content of open pore volume. The manufacture method typically involves vapor phase sintering of a green form of metal oxide feed to form a porous sintered metal oxide material. Prior to the vapor phase sintering, the metal oxide is typically in a fine particulate form that has been compressed or otherwise preformed into the desired green form. After the vapor phase sintering, at least a portion of the precursor metal oxide is then reduced to metallic form to prepare the porous metal-containing material. It should be noted that the sintered metal oxide and the method of making the sintered metal oxide are within the scope of the present invention, as is the reduction of the sintered metal oxide to form the porous metal-containing material.
A particularly surprising and advantageous aspect of the manufacture method is that the method may be performed with very little, if any, shrinkage occurring between the metal oxide green form and the final porous metal-containing material. Not to be bound by theory, but to aid in the understanding of the invention, this low level of shrinkage is believed to be at least partially due to manufacture of the sintered metal oxide by vapor phase sintering. Typically, the shrinkage occurring between the metal oxide green form and the porous metal-containing material will be smaller than about 15 volume percent, preferably smaller than about 10 volume percent, and most preferably smaller than about 5 volume percent. In many instances, shrinkage may be smaller than about 2 volume percent. Furthermore, the low shrinkage values noted apply to each of the steps of vapor phase sintering (metal oxide green form vs. sintered metal oxide) and reduction (sintered metal oxide vs. porous metal-containing material), as well as to the overall process (metal oxide green form vs. porous metal-containing material). As will be appreciated, when reference is made herein to shrinkage, the reference is relative to the bulk volume of the relevant material, which includes pore volume.
Furthermore, high porosities in the porous metal-containing material may be achieved typically without the use of organics. Organics may, however, be used if desired. For example, to make materials of extremely high porosity, such as larger than about 80 volume percent, a polymer may be mixed with the metal oxide powder to serve as a surrogate pore former. Subsequent removal of the polymer will result in a higher pore volume. In this way, porosities of even larger than about 90 percent may be possible. When polymers are added, they are preferably in the form of small spheres, which may be of any convenient size. Typical sizes are from about 1 micron, or smaller, to about 200 microns, although the smaller sizes are preferred for most applications. The polymer is typically removed by thermal decomposition/volatilization or dissolution in a solvent prior to the vapor phase sintering step.
When the porous metal-containing material includes multiple metals, the metal oxide green form could include a mixture of different metal oxide powders. Preferably, however, the metals would be present in the form of a single powder of a complex metal oxide.
The vapor phase sintering of the metal oxide feed occurs at conditions under which at least one volatile metal-containing component is present at a significant vapor pressure. This is often accomplished through inclusion of at least one reactive gas component in the atmosphere during sintering. The reactive gas component reacts with the metal oxide to form one or more volatile metal-containing components which are responsible for significant mass transport in the vapor phase during the sintering operation. Although other transport mechanisms may also contribute to mass transport, the vapor phase transport should be the dominant mechanism, so that vapor phase transport is the controlling mechanism for mass transport. The reactive gas component may be any component that exists in the vapor phase at the conditions of the vapor phase sintering. Preferred reactive gas components are HCl and other halide gases, although other reactive gas components, as are now or hereafter known in the art for vapor phase sintering, could be used instead.
The temperature at which the vapor phase sintering is conducted will depend upon the specific metal oxide feed and the desired properties in the final porous metal-containing material. In most instances, however, the temperature will be in a range of from about 500xc2x0 C. to about 1700xc2x0 C. For a given material, the use of a higher temperature will typically result in a larger mean pore size and a lower temperature will result in a smaller mean pore size in the porous metal-containing material. Also, the size and uniformity of the pores may be controlled to some degree by the particulate characteristics of metal oxide feed. The use of smaller starting metal oxide particles tends to result in smaller pores in the porous metal-containing material. The use of more uniformly sized metal oxide particles tends to result in more uniform pore sizes in the porous metal-containing material.
As noted, the vapor phase sintering is often conducted in an atmosphere including one or more reactive gas components. The atmosphere during vapor phase sintering may also include, if desired, an inert constituent, such as argon. For many application, the atmosphere in which the vapor phase sintering takes place should be neither substantially oxidizing nor substantially reducing in character. Rather, it is often preferred that the atmosphere promote primarily only reactions with the metal oxide feed that contribute to vapor phase mass transport during the vapor phase sintering.
The reduction of the precursor, following the vapor phase sintering, is conducted in any suitable reducing environment. For example, the reducing atmosphere may include a reducing gas, such as hydrogen gas. The hydrogen gas may be mixed with an inert gas, such as nitrogen. As another example, the reduction may be conducted in a vacuum, either with or without the presence of a reducing gas. The temperature during the reduction may be any convenient temperature, but will typically be lower than the temperature during the vapor phase sintering.
The present invention also includes uses for the porous metal-containing materials and products incorporating or made, at least in part, by further processing of the metal-containing material. For example, the porous metal-containing material may be used as filter elements, electrode materials in electrochemical cells, heat exchange elements, catalysts, combustion substrates and light-weight structural members.